1. Field of the Invention
The present invention relates to a color image reading apparatus and, more particularly, to a color image reading apparatus, which can read color image information on an original surface with high precision using a simple monolithic 3-line sensor by correcting the asymmetric spacings of a plurality of color-separated light beams in the sub scanning direction on the surface of a light-receiving means caused by different focusing positions (imaging positions) arising from different wavelengths of diffracted light beams color-separated by a color-separation means comprising a reflection or transmission type linear blazed diffraction grating, and is suitable for, e.g., a color scanner, color facsimile, and the like.
2. Related Background Art
Conventionally, various apparatuses for digitally reading color image information using output signals from a line sensor by imaging color image information on an original surface on the surface of the line sensor (CCD) via an optical system have been proposed.
FIG. 1 is a schematic view showing principal parts of an optical system of a conventional color image reading apparatus. In FIG. 1, when a light beam originating from a color image on an original surface 64 is focused and imaged by an imaging lens 69 on the surface of a line sensor (to be described below), the light beam is color-separated into three colors, i.e., red (R), green (G), and blue (B) via a 3P prism 60, and these color-separated light beams are respectively guided onto the surfaces of line sensors 61, 62, and 63. Color images formed on the surfaces of the line sensors 61, 62, and 63 are line-scanned in the sub scanning direction, thus reading images in units of colors.
FIG. 2 is a schematic view showing principal parts of an optical system of another conventional color image reading apparatus. In FIG. 2, when a light beam originating from a color image on an original surface 64 is focused and imaged by an imaging lens 79 on the surface of a line sensor (to be described later), the light beam is split into three light beams corresponding to the three colors via two color-separation beam splitters 70 and 71 each added with a wavelength selective transmission film having dichroism.
Color images based on the three color light beams are respectively imaged on the surface of a so-called monolithic 3-line sensor 72 arranged on a single substrate surface. The color images are line-scanned in the sub scanning direction to read images in units of colors.
FIG. 3A is an explanatory view of the monolithic 3-line sensor 72 shown in FIG. 2. As shown in FIG. 3A, the monolithic 3-line sensor 72 has three parallel line sensors (CCDs) 65, 66, and 67 which are placed on a single substrate surface and are spaced by a finite distance. Color filters (not shown) based on the respective color light beams are mounted on the surfaces of these line sensors.
Spacings S.sub.1 and S.sub.2 between adjacent line sensors 65, 66, and 67 are normally set to fall within the range of about 0.064 to 0.2 mm under various manufacturing conditions. On the other hand, pixel widths W.sub.1 and W.sub.2 of one pixel 68 are set to be, e.g., in the neighborhood of 7 .mu.m.times.7 .mu.m or 10 .mu.m.times.10 .mu.m (see FIG. 3B).
The color image reading apparatus shown in FIG. 1 requires three independent line sensors, and requires high precision. In addition, the apparatus shown in FIG. 1 requires the 3P prism which is hard to manufacture. Hence, the entire apparatus becomes complicated and expensive. Furthermore, alignment between the imaging light beams and line sensors must be independently done three times, resulting in cumbersome assembly and adjustment.
On the other hand, in the color image reading apparatus shown in FIG. 2, if x represents the thickness of each of the beam splitters 70 and 71, the distance between adjacent lines of the line sensors is 2.sqroot.2x. If the preferred distance between adjacent lines of the line sensors in terms of manufacture is about 0.1 to 0.2 mm, the thickness x of the beam splitter 70 or 71 becomes about 35 to 70 .mu.m.
In general, it is very hard to manufacture a beam splitter which has such small thickness and can optically maintain high flatness. When a beam splitter with such thickness is used, the optical performance of a color image to be formed on the line sensor surface lowers.
On the other hand, as shown in FIGS. 4A and 4B, the distances S.sub.1 and S.sub.2 between lines of the two line sensors 65 and 67 with respect to the central line sensor 66 of the monolithic 3-line sensor are normally set to be equal to each other in the opposite directions and to be integer multiples of the pixel size W.sub.2 (see FIG. 4B) in the sub scanning direction for the reason given below.
That is, as shown in FIGS. 4A and 4B, when the monolithic 3-line sensor reads color images using a normal imaging optical system 89 alone, the reading positions on the original surface 64 that can be simultaneously read by the three line sensors 65, 66, and 67 are three different positions 65', 66', and 67', as shown in FIG. 4A.
For this reason, three color signal components (R, G, B) for an arbitrary position on the original surface 64 cannot be simultaneously read. Hence, after the images are read by the three line sensors, they must be aligned and synthesized.
In this processing, when the distances S.sub.1 and S.sub.2 between adjacent lines of the three line sensors are set to be integer multiples of the pixel size W.sub.2 and corresponding redundant line memories are used, for example, the G and R signals (signal components based on the G and R color light beams) are delayed with respect to the B signal (a signal component based on the B color light beam), thereby relatively easily obtaining a synthesized signal component of three colors.
For this reason, the distances S.sub.1 and S.sub.2 of the two line sensors 65 and 67 with respect to the central line sensor 66 of the 3-line sensor are set to become integer multiples of the pixel size W.sub.2 in the sub scanning direction.
However, upon assigning redundant line memories in correspondence with the line separations of the 3-line sensor in the above-mentioned color image reading apparatus, a plurality of arrays of expensive line memories must be arranged, and this results in very high cost and a complicated apparatus as a whole.
In general, the distances S.sub.1 and S.sub.2 between adjacent lines of the three line sensors preferably assume equal values to attain easy semiconductor processes.
As still another color image reading apparatus, the following apparatus has proposed by U.S. Pat. No. 5,223,703. In this apparatus, as shown in FIG. 5, a monolithic 3-line sensor is used as a light-receiving means (light-receiving element) 104, and a reflection type linear blazed diffraction grating 103 serving as a color-separation means is inserted in the imaging optical path to be spaced from the exit pupil of an imaging lens (projection lens) 109 in the direction of the surface of the light-receiving means 104. Color separation is attained using reflection and diffraction, and color image information for one line on the original surface 64 is color-separated and imaged on the surface of the 3-line color sensor 104, thereby reading the color image information.
The color image reading apparatus using the reflection type linear blazed diffraction grating as the color-separation means suffers the following problem.
Limitations on the peak wavelengths, half-width wavelengths, overlap amounts of the colors, and the like are imposed on the reading wavelength ranges of the individual colors color-separated by a color-separation system. For example, assuming that the wavelength characteristics shown in FIG. 6 are ideal for a reading system, the angles of .+-.1st-order diffracted light components with respect to 0-order diffracted light do not match each other independently of the pitch of the reflection type linear blazed diffraction grating, and asymmetry remains. For this reason, the spacings between adjacent color light beams (color light spacings) on the surface of the 3-line sensor differ.
Hence, the conventional apparatus requires manufacture of a special sensor which has asymmetric line spacings (sensor spacings) in the sub scanning direction of the monolithic 3-line sensor, i.e., does not have general equal line spacings.
In order to solve such problem, Japanese Patent Application Laid-Open No. 8-223359 (corresponding to U.S. application Ser. No. 596,623 and EP Publication No. 0731598) has proposed an apparatus in which a dichroic mirror having at least two reflection surfaces is inserted in the optical path between a blazed diffraction grating and light-receiving means.
In the above reference, the R and B color light beams are reflected by the first reflection surface of the dichroic mirror, and the G color light beam is reflected by the second reflection surface of the dichroic mirror, thereby producing optical path differences between the R and B color light beams, and the G color light beam, and shifting the imaging positions of the three color light beams on the light-receiving means.
However, with this method, since the shift amount of the color light beams is determined by the spacing between the two reflection surface, it is hard to adjust the imaging positions.